A hydraulic shovel 100 will be described with reference to FIG. 5 of the accompanying drawings. A lower caterpillar tractor 21 (called "traveling undercarriage") is movable back and forth by a driving motor 24. An upper revolvable member 23 (called "upper revolvable turret", "upper vehicle", or "main vehicle") is mounted on the caterpillar tractor 21 via a turntable 22, and is turned about the vertical axis of the turntable 22 by a turning motor 25. The upper vehicle 23 comprises an excavating machine 30, a machine cab 26, an operator cabin 27, and a counterweight 28.
The excavating machine 30 includes a boom 31, a digging arm 33, a bucket 35, a boom actuator 32, an arm actuator 34, and a bucket actuator 36. One end of the boom 31 is pivotally attached to a bracket (not shown), fixed to the upper vehicle 23, and is movable vertically in response to the operation of the boom actuator 32. One end of the digging arm 33 is pivotally attached to the distal end of the boom 31 and is movable vertically in response to the operation of the arm actuator 34. The bucket is pivotally attached to the lower (distal) end of the digging arm 33 and is pivoted in response to the operation of the bucket actuator 36.
The motors 24 and 25 and the actuators 32, 34, and 36 are operated independently of each other or in various combinations with each other by a control circuit (not shown) in order to perform excavation. At the present time, there is a strong demand for a control circuit which can more efficiently control the combined operations of the foregoing members.
Japanese Patent Laid-Open Publication 1-250531 exemplifies a device for improving the efficiency of combined operations of an excavating machine. The device will be described referring to FIG. 6. When the pilot valve 50 is operated in the direction U in order to contract the arm actuator 55, a pilot pressure is applied to the pilot conduit 50A. In this state, if the pilot valve 51 has also been operated in the direction U in order to contract the boom actuator 56, a pilot pressure is introduced from the pilot valve 51 to the pilot port 52A of the selector valve 52, so that the selector valve 52 is switched over from its position a to its position b. The pilot pressure from pilot valve 51 is also applied to a control port of control valve 58, which controls the boom actuator 56.
The pilot circuit is formed by the selector valve 52 and the reducing valve 53. The pilot pressure applied via the pilot conduit 50A to the reducing valve 53 is reduced to a predetermined pressure by the reducing valve 53, and the reduced pressure acts on the control valve 57. If both the pilot valves 50 and 51 are operated to an equal extent, the pilot pressure in the pilot conduit 50A downstream of the reducing valve 53 is lower than the pilot pressure in the pilot conduit 51A. Therefore, an open area of the spool of the control valve 57 is smaller than an open area of the control valve 58.
As a result, an increased amount of pressurized oil flows to the control valve 58 via the conduit 54, so that the difference between the amount of pressurized oil applied to the arm actuator 55 and the amount of pressurized oil applied to the boom actuator 56 becomes smaller. Therefore, both the actuators 55 and 56 are contracted at substantially equal speeds, which means that they operate simultaneously at equal speeds.
There is another related art, e.g., Japanese Patent Laid-Open Publication No. 8-93000. This device of that publication will be described with reference to FIGS. 7 and 8. When the driving motor 24 and the arm actuator 34 are operated in combination, e.g., when the shift lever 48 is set to the forward position, the reducing valve 48b of the driving pilot valve 48a is activated. The pilot pressure for switching the driving control valve 42 from its neutral position a over to its position b acts not only on the operation part 42b of the driving control valve 42 but also on the operation part 45b of the arm control valve 45 via the shuttle valve 43 and the branch pilot conduit 41a. Therefore, the piston 47 in the arm control valve 45 is moved to the predetermined extent L in the arrow direction as shown in FIG. 8.
It is assumed here that the reducing valve 49b of the arm pilot valve 49a is operated with the arm operating lever 49 set to the excavation position in order to change the arm control valve 45 over to its position a from its neutral position n, and that the pilot pressure from the reducing valve 49b is applied to the operation part 45a of the arm control valve 45 via the pilot conduit 44a. The spool 46 (FIG. 8) of the arm control valve 45 is controlled so as not to operate with the full stroke, which reduces the open area of the spool 46. This is because the piston 47 has already been shifted by the predetermined extent L in the arrow direction as shown in FIG. 8. Therefore, the amount of pressurized oil applied to the arm actuator 34 is reduced, thereby enabling the operating speed of the traveling motor 24 and that of the arm actuator 34 to be matched.
However, there is a problem in that the operating speeds of the two actuators are not matched when they are operated in combination in order to cause the hydraulic shovel to do various jobs. This is because more pressurized oil tends to flow from a hydraulic pump to the actuator having a lower load. This problem is remarkable when a boom of the hydraulic shovel is moved and the upper vehicle is turned (so-called "hoist and turn") at the same time, a combination which is frequently performed during the operation of the hydraulic shovel. In such a case, the actuator for turning the upper vehicle has a lower load than that of the actuator for vertically moving the boom.
During the combined operation of the boom lifting and the upper vehicle turning, a larger amount of pressurized oil is supplied from the hydraulic pump to the actuator having the lower load, so that the upper vehicle is turned at an increased speed while the boom is moved at a reduced speed, i.e., the operation speed of the boom does not match the operation speed of the turning of the upper vehicle. This is because the two actuators are connected to the control circuit in parallel. In order to overcome this problem, the operator has to adjust an operation stroke of the turning lever in order to reduce the flow of pressurized oil from the hydraulic pump to the turn actuator, and to reduce the open area of the spool of the turn control valve so that the operation speeds of the two actuators can be matched. Thus, the operator is required to adjust the operation extent of the turn lever as well as performing the combined operation of the boom and the upper vehicle, which is complicated and troublesome.
In the pilot operation circuit described in Japanese Patent Laid-Open Publication No. 1-250531, the actuators 55 and 56 are usually operated at substantially the same speed during the combined operation, which is a rather slow speed. Further, there is another problem in that the open area of the spool of the arm control valve 57 is continuously regulated during the combined operation, and this regulation cannot be cancelled.
The pilot operation circuit of Japanese Patent Laid-Open Publication No. 8-93000 is effective for combined operations of the arm and the upper vehicle on an uphill gradient. However, the open area of the spool of the arm control valve is continuously regulated, and this regulation cannot be cancelled.
Both of the foregoing related arts are rather difficult to apply to the combined operations of the boom and the upper vehicle when excavated soil has to be quickly loaded onto a dump truck by operating the hydraulic shovel and turning the upper vehicle at the same time.